Aircraft transparency having chemically tempered lithia-alumina-silica containing glass and method of making the chemically tempered glass

ABSTRACT

An aircraft transparency in a glass piece, wherein the glass piece includes a chemically tempered first major surface and a chemically tempered opposite second major surface, a first case depth begins at the first major surface, a second case depth begins at the second major surface, and a tensile stress zone is within the glass piece between the end points of the first and the second case depths. The glass between the end points of the first and second case depth has a glass composition including: 
                                   Ingredient   Percent by weight               SiO 2     60 to 75;         Al 2 O 3     18 to 28; and         Li 2 O   3 to 9, and                               
the glass has at least one of the following properties (a) a log 10 viscosity temperature of at least 1413° F. and (b) a liquidus temperature of at least 2436° F.

CROSS REFERENCE APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 10/956,500 filed on Oct. 1, 2004 in the names of Larry J.Shelestak, George B. Goodwin, Amarendra Mishra and James M. Baldauff andtitled LITHIA-ALUMINA-SILICA CONTAINING GLASS COMPOSITIONS AND GLASSESSUITABLE FOR CHEMICAL TEMPERING AND ARTICLES MADE USING THE CHEMICALLYTEMPERED GLASS. This application claims the benefit of U.S. ProvisionalPatent Application Ser. No. 60/514,136, filed Oct. 24, 2003. applicationSer. Nos. 10/956,500 and 60/514,136 are incorporated by reference hereinin their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an aircraft transparency having chemicallytempered lithia-alumina-silica containing glass and to a method ofmaking the chemically tempered glass.

2. Technology and Utility Discussion

Chemical strengthening (or “chemical tempering”) of glass involves anexchange of ions near the surface of the glass, e.g. a glass articlewith ions from an external source, typically a molten inorganic saltbath, to generate a zone near the surface of the glass which is in astate of compression relative to the interior portions of the glass.There are two types of ion exchange strengthening which differsubstantially in theory and operation. The first type of ion exchangetreatment is carried out above the strain point of the glass and has asits objective the alteration of the glass composition at the surface tolower the thermal coefficient of expansion in the surface layer. As theglass is cooled, a compressive stress develops at the surface of theglass due to the expansion differential. The first type of ion exchangestrengthening is discussed in U.S. Pat. No. 2,779,136. The second typeof ion exchange strengthening is characterized by treatment below thestrain point of the glass. In the second type, the surface compressionis generated by substituting large ions from an external source (e.g., amolten salt bath) for smaller ions in the glass. Typically, thesubstitution is sodium or potassium ions for lithium ions in alithia-alumina-silica glass, e.g. of the type discussed in U.S. Pat.Nos. 3,218,220; 3,752,729; 3,900,329; 4,156,755 and 5,928,793, orpotassium ions for sodium ions in a soda-alumina-silica glass, e.g. ofthe type discussed in U.S. Pat. Nos. 3,485,702; 3,752,729; 4,055,703,and 4,015,045.

Of the two types of ion exchange strengthening, the second (below thestrain point) type is preferred for large-scale commercial use becausemaintaining the glass below its strain point avoids undesirabledistortion defects in the glass. Of the two types of glass compositions,the lithia-alumina-silica glass compositions are preferred over thesoda-alumina-silica glass compositions, and the preferred ion exchangeis the sodium ion for the lithium ion. The lithia-alumina-silica glasscompositions and the exchange of the sodium ion for the lithium ion arepreferred because a deeper depth of ion exchange (“case depth”) in ashorter period of time at lower temperatures can be obtained.

An appreciation of the selection of the lithia-alumina-silica glasscompositions and the exchange of the sodium ion for the lithium ion ishad when a comparison of the ionic crystal radius of the sodium, lithiumand potassium ions is made. The sodium atom has an ionic crystal radiusof about 95 picometers (“pm”), the lithium atom has an ionic crystalradius of 60 pm and the potassium ion has an ionic crystal radius ofabout 133 pm. The lithium ion having a smaller ionic crystal radius thanthe sodium ion requires less energy to displace from the glass than thesodium ion, and the sodium ion having a smaller ionic crystal radiusthan the potassium ion requires less energy than the potassium ion todisplace the lithium ion from the glass.

As mentioned above, lithia-alumina-silica glasses are available, e.g.disclosed in U.S. Pat. Nos. 3,218,220; 3,752,729; 3,900,329; 4,156,755and 5,928,793. As can be appreciated by those skilled in the art, itwould be advantageous to provide additional lithia-alumina-silica glasscompositions for ion exchange strengthening, and in particularlithia-alumina-silica glass compositions that can attain a higherstrength than the presently available lithia-alumina-silica glasscompositions.

SUMMARY OF THE INVENTION

This invention relates to an aircraft transparency including, amongother things, a glass piece. The glass piece includes, among otherthings, a chemically tempered first major surface and an opposite secondmajor surface; a case depth, wherein the case depth is defined as adistance from the first major surface of the glass piece toward thesecond major surface, the distance ending at a position within the glasspiece at which the glass piece has is zero stress, and a tensile stresszone within the glass piece at a depth from the first major surfacegreater than the distance of the case depth from the first majorsurface. The glass in the tensile stress zone has a glass composition,including, among other things:

Ingredient Percent by weight SiO₂ 60 to 75; Al₂O₃ in the range selectedfrom the group of 18 to 29; 18 to 28, 19 to 28.5, and 20 to 25; Li₂O 3to 9; and ZrO₂ 0 to 3;

where percent by weight (“wt %”) of Al₂O₃+ZrO₂ is in the range selectedfrom the group of 18 to 28 wt %, 19 to 27 wt %, 20 to 26 wt %, 18 to 29wt %, 19 to 28.5 wt % and 20 to 26 wt %, and wherein the glass in thetensile stress zone has at least one of the following properties: (a) alog 10 viscosity temperature of at least 1280° F. (694° C.) and (b) aliquidus temperature of at least 2350° F. (1288° C.).

Further, this invention relates to an aircraft transparency including,among other things, a glass piece. The glass piece includes, among otherthings, a chemically tempered first major surface and a chemicallytempered opposite second major surface; a first case depth and a secondcase depth, wherein the first case depth is defined as a first distancefrom the first major surface of the glass piece toward the second majorsurface, the first distance ending a first position within the glasspiece at which there is zero stress and the second case depth is definedas a second distance from the second major surface of the glass piecetoward the first major surface, the second distance ending a secondposition within the glass piece at which there is zero stress, whereinthe first position is spaced from the second position, and a tensilestress zone defined as interior glass. The interior glass is within theglass piece between the first and the second positions, and has a glasscomposition comprising:

Ingredient Percent by weight SiO₂ 60 to 75; Al₂O₃ 18 to 28; and Li₂O 3to 9;

wherein the glass composition has at least one of the followingproperties (a) a log 10 viscosity temperature of at least 1413° F. and(b) a liquidus temperature of at least 2436° F.

Still further, this invention relates to a method of chemicallystrengthening a glass piece by, among other things, providing a glasspiece having a predetermined shape and thickness and a compositionincluding, among other things:

Ingredient Percent by weight SiO₂ 60 to 75; Al₂O₃ 18 to 28; Li₂O 3 to 9;Na₂O 0 to 3; K₂O 0 to 0.5; CaO 0 to 3; MgO 0 to 3; SO₃ 0 to 0.20; Totaliron expressed as Fe₂O₃ 0 to 1.25; ZrO₂ 0 to 3; Total tin expressed asSnO₂ 0 to 0.70; TiO₂ 0 to 5; P₂O₅ 0 to 1.75; ZnO 0 to 1.25; and B₂O₃ 0to 1.75;

where weight percent (“wt %”) of CaO+MgO is in the range of 0 to 6 wt %,wt % of Al₂O₃+ZrO₂ is in the range of 18 to 28 wt %, and wt % ofNa₂O+K₂0 is in the range of 0.05 to 3.00 wt %, wherein the glass piecehas a log 10 viscosity temperature of at least 1413° F. and a liquidustemperature of at least 2435° F.; submerging the glass piece in a moltensodium nitrate bath heated to a temperature of greater than 300° F. forat least 8 hours to provide the glass piece with a case depth in therange of 7 to 16.5 mils, and removing the glass piece from the bath.

DETAILED DESCRIPTION OF THE INVENTION

It is understood that in the following discussion all numbers expressingdimensions, physical characteristics, and so forth, used in thespecification and claims are to be understood as being modified in allinstances by the term “about”. Accordingly, unless indicated to thecontrary, the numerical values set forth in the following specificationand claims can vary depending upon the desired properties sought to beobtained by the present invention. At the very least, and not as anattempt to limit the application of the doctrine of equivalents to thescope of the claims, each numerical parameter should at least beconstrued in light of the number of reported significant digits and byapplying ordinary rounding techniques. Moreover, all ranges disclosedherein are to be understood to encompass any and all subranges subsumedtherein. For example, a stated range of “1 to 10” should be consideredto include any and all subranges between and inclusive of the minimumvalue of 1 and the maximum value of 10; that is, all subranges beginningwith a minimum value of 1 or more and ending with a maximum value of 10or less, e.g., 5.5 to 10. Additionally, any reference “incorporated byreference” means incorporation of the entire reference.

In the discussion of thermal properties of glass compositions or glassesas they relate to temperatures for melting glass batch materials, finingthe molten glass and/or forming a glass ribbon from the fined moltenglass, the following terms are usually used and have the followingmeanings. The terms “melting temperature” and “T_(M)” mean thetemperature of the glass at which the viscosity of the glass is log 2 or100 poise; the terms “forming temperature” and “T_(F)” mean thetemperature of the glass at which the viscosity of the glass is log 4,or 10,000 poise, and the terms “liquidus temperature” and “T_(L)” meanthe temperature at which minute crystals are in equilibrium with theliquid phase of the glass melt. The difference between T_(F) and T_(L)is referred to as the “forming window”, “working range” or “ΔT” and is acommon measure of the crystallization potential of a given meltcomposition. The lower the ΔT, in other words the smaller the differencebetween the forming temperature and the liquidus temperature, thegreater the crystallization potential. It is understood that the aboveterms when used in the discussion of the nonlimiting embodiments of theinvention and in the claims shall have the above meaning.

The term “strain point” as used herein means the temperature at whichstresses in glass having a log viscosity of 14.5, or 10^(14.5) poise arerelieved after 4 hours. The term “annealing point” as used herein meansthe temperature at which stresses in glass having a log viscosity of 13,or 10¹³ poise are relieved in 15 minutes. The term “case depth” meansthe distance measured from a surface of a glass piece to a position inthe interior of the glass piece at which there is zero stress in theglass piece. The glass within the case depth is also referred to as“glass in the compression zone.” As can be appreciated by those skilledin the art, the case depth is not always equal to the ion exchange depthin the glass piece. The ion exchange depth is the depth in the glasspiece where the concentration of the new, exchanged ion is about equalto the concentration of the new exchanged ion in the interior of theglass piece. The surface can be any surface of the glass piece exposedto the molten salts of the ion exchange bath, e.g. but not limiting tothe invention the sodium nitrate molten bath. In the general practice ofchemical tempering glass, a glass piece is submerged in a sodium nitratemolten bath. In this instance, all the surfaces of the glass pieceexposed to the molten bath have a case depth. Further, the compositionof the glass piece within the compression zone has a compositiondifferent from portions of the glass piece outside the compression zonealso referred to as “glass in the tensile stress zone”. Stated anotherway, the “glass in the tensile stress zone” is the internal portion ofthe glass piece spaced from the surface of the glass piece a greaterdistance than the case depth. The reason for the differences in thecompositions in the tensile stress zone of the glass piece and in thecompression zone of the glass piece is that the portion of the glasspiece within the compression zone has more sodium ions than the portionof the glass piece in the tensile stress zone. It is understood that theabove terms when used in the discussion of the nonlimiting embodimentsof the invention and in the claims shall have the above meanings.

In the following discussion of the nonlimiting embodiments of the glasscompositions of the invention, the ingredients are given in weightpercent (“wt %”); however, as can be appreciated, the invention is notlimited thereto and the ingredients can be given in any dimension, e.g.moles or mole percent, which identifies the quantity of the ingredientin the batch materials or the quantity of the ingredient, e.g. oxide inthe glass composition. Further in the following discussion the wt %range of an ingredient or material in the glass identified as an“impurity” or a “tramp” means that the impurity or tramp ingredient ormaterial present in the glass within the wt % range is an impurity ortramp ingredient or material whether purposely added to the batchmaterials or present in the batch materials as an impurity or trampmaterial or ingredient.

The invention and/or the practice of the invention, but not limitingthereto, will be discussed using a generic glass system that includeslithia (Li₂O), alumina (Al₂O₃), silica (SiO₂) and alkali and alkalineearth metal oxides, e.g. a glass system that includes Li₂O, Al₂O₃ andSiO₂, and selected ones and/or selected amounts of Na₂O, MgO, SnO₂,P₂O₅, B₂O₃ and K₂O, and optionally other metal oxides either asadditions or tramp materials, e.g. but not limiting the inventionthereto, CaO, SO₃, Fe₂O₃, TiO₂, MnO₂, ZrO₂ and ZnO.

SiO₂, Al₂O₃, B₂O₃, P₂O₅, ZrO₂ and SnO₂ when present in the glass arenetwork formers. It can be inferred that as SiO₂, which is the largestoxide component of the glass composition in terms of weight percent, isreduced in a given composition of this type, the melt viscosity and theresulting log 4 forming temperature drops and vice versa. SiO₂ isincluded to provide a glass that has high temperature stability andchemical durability. Low concentrations of SiO₂, e.g. lower than 50 wt%, decrease the durability of the glass whereas higher concentrations,e.g. higher than 80 wt %, require higher melting temperatures and longermelting times. In the practice of the invention, nonlimiting embodimentsof the invention, include, but are not limited to, SiO₂ in a rangeselected from the group of 60 to 75 wt %, 62 to 72 wt % and 64 to 72 wt%.

Al₂O₃ is present in the glass system to increase the glass strengthafter ion exchange and to promote the ion-exchange propensity. HighAl₂O₃ concentrations in glass, e.g. above 7 wt %, increase the strainpoint of the glass, which allows performing the ion exchange at highertemperatures, e.g. above 752° F. (400° C.), resulting in faster ionexchange rates, deeper case depth and higher ion exchange penetration.Higher concentrations of Al₂O₃, e.g. higher than 30 wt %, also increasethe melting temperatures of the glass and result in a glass having pooracid durability. Concentrations of Al₂O₃ in glass lower than 7 wt %decrease the case depth. In the practice of the invention, nonlimitingembodiments of the invention include, but are not limited to, Al₂O₃ in arange selected from the group of 18 to 29 wt %, 18 to 28 wt %, 19 to28.5 wt % and 20 to 25 wt %.

B₂O₃ and P₂O₅ are network formers and are optionally added to the batchmaterials to lower the log 2 melting viscosity of the glass. B₂O₃ andP₂O₅ each in amounts below 0.05 wt % in the glass are considered animpurity or a tramp ingredient. In the practice of the invention,nonlimiting embodiments of the invention include, but are not limitedto, B₂O₃ in a range selected from the group of 0 to 1.75 wt %, e.g. nogreater than 1.75 wt %; 0 to 1.5 wt %, e.g. no greater than 1.5 wt %; 0to 1.25 wt %, e.g. no greater than 1.25 wt %; 0 to 2.5 wt %, e.g. nogreater than 2.5 wt %; 0 to 2.25 wt %, e.g. no greater than 2.25 wt %,and 0 to 2 wt %, e.g. no greater than 2 wt %. Further, in the practiceof the invention, nonlimiting embodiments of the invention, include butare not limited to, P₂O₅ in a range selected from the group of 0 to 1.75wt %, e.g. no greater then 1.75, preferably in the range of 0 to 1.5 wt%, e.g. no greater than 1.5 wt % and more preferably in the range of 0to 1.25 wt %, e.g. no greater than 1.25 wt %.

ZrO₂, like B₂O₃ and P₂O₅, is a network former and is optionally added tothe batch materials to lower the log 2 melting viscosity of the glass.ZrO₂ in amounts up to about 3 wt % increases the glass strength andlowers the melting temperature of the glass. ZrO₂ above 10 wt %increases the crystallization temperature and lowers the working rangeof the glass. In the practice of the invention, nonlimiting embodimentsof the invention include, but are not limited to, ZrO₂ in a rangeselected from the group of 0 to 3 wt %, e.g. no greater than 3 wt %; 0to 2 wt %, e.g. no greater than 2 wt %, and 0 to 1.75 wt %, e.g. nogreater than 1.75 wt %.

The sum of wt % of Al₂O₃+ZrO₂ (hereinafter also referred to as “NF”) isanother parameter that describes the compositional envelope of a givenglass. A lower value of Al₂O₃+ZrO₂ will typically result in a lowercrystallization potential; a greater value of Al₂O₃+ZrO₂ will typicallyresult in a higher crystallization potential for a given melt. In thepractice of the invention, nonlimiting embodiments of the inventioninclude, but are not limited to, NF in a range selected from the groupof 18 to 28 wt %; 19 to 27 wt %; 20 to 26 wt %; 18 to 29 wt %; 19 to28.5 wt %, and 20 to 26 wt %.

As discussed above, SnO₂ is a network former. During heating, SnO₂ givesup oxygen to form SnO as part of the fining process. As can beappreciated by those skilled in the art, the “total tin” content of theglass compositions disclosed herein is expressed in terms of SnO₂ inaccordance with standard analytical practice, regardless of the form oftin actually present in the glass. In the practice of the invention,nonlimiting embodiments of the invention include, but are not limitedto, SnO₂ in a range selected from the group of 0 to 0.7 wt %, e.g. nogreater than 0.7 wt %; 0 to 0.6 wt %, e.g. no greater than 0.6 wt %, and0 to 0.55 wt %, e.g. no greater than 0.55 wt %.

CaO and MgO are network modifiers and fluxes that aid in the melting ofthe network formers in the batch materials. It is the usual practice torefer to the sum of the wt % of CaO and MgO as RO, i.e. RO equals(CaO+MgO). Increasing RO will reduce the working window or ΔT; moreparticularly, increasing RO increases the fluidity of the resultingmelt, i.e. decreases the viscosity and the forming temperature of themelt, and increases the crystallizability of the resulting melt, i.e.increases its liquidus temperature. In the practice of the invention itis preferred to reduce the wt % of the CaO because CaO slows down thelithium and sodium ion exchange thereby slowing down the rate of ionexchange. In the practice of the invention, nonlimiting embodiments ofthe invention include, but are not limited to, CaO in a range selectedfrom the group of 0 to 3 wt %, e.g. no greater than 3 wt %; 0 to 2 wt %,e.g. no greater than 2 wt %, and 0 to 1.75 wt %, e.g. no greater than1.75 wt %.

MgO has less effect than CaO in blocking the lithium and sodium ionexchange. In the practice of the invention, nonlimiting embodiments ofthe invention include, but are not limited to, MgO in a range selectedfrom the group of 0 to 3 wt %, e.g. no greater than 3 wt %; 0 to 2.75 wt%, e.g. no greater than 2.75 wt %, and 0 to 2.5 wt %, e.g. no greaterthan 2.5 wt %. To aid in the melting of the network formers whileproviding an acceptable working range, e.g. above 10° C., in thepractice of the invention, nonlimiting embodiments of the inventioninclude, but are not limited to, RO in a range selected from the groupof 0 to 6 wt %, e.g. no greater than 6 wt %; 0 to 5 wt %, e.g. nogreater than 5 wt %, and 0 to 4 wt %, e.g. no greater than 4 wt %.

In the practice of the invention, ZnO can be used in place of or toreduce the amount of MgO and/or CaO. As can be appreciated by thoseskilled in the art of glass making, ZnO is not an alkaline earth but Znhas the +2 oxidation state and acts in a similar manner as Mg and Caions. In the practice of the invention, nonlimiting embodiments of theinvention include, but are not limited to, ZnO in a range selected fromthe group of 0 to 1.25 wt %, e.g. no greater than 1.25 wt %; 0 to 1 wt%, e.g. no greater than 1 wt %, and 0 to 0.75 wt %, e.g. no greater than0.75 wt %.

Li₂O provides the Li ions, which are displaced from the glass by thesodium ion from an external source, e.g. a molten bath of sodiumnitrate. It is preferred in the practice of the invention to have asufficient and uniform distribution of Li ions in the glass so that theion exchange of the Na ion for the Li ion provides a sufficient anduniform distribution of replacement Na ions in the glass from theexternal source. In the practice of the invention, nonlimitingembodiments of the invention include, but are not limited to, Li₂O in arange selected from the group of 3 to 9 wt %: 4 to 7.5 wt %, and 4.5 to7 wt %.

Sodium oxide is a flux aiding in the melting of the batch materials bylowering the glass viscosity. A tramp amount of Na₂O in the glass isequal to or less than 0.05 wt %. In the practice of the invention,nonlimiting embodiments of the invention include, but are not limitedto, Na₂O in a range selected from the group of 0 wt % or a tramp amountto 3 wt %, e.g. no greater than 3 wt %; 0 wt % or a tramp amount to 2 wt%, e.g. no greater than 2 wt %, and 0 wt % or a tramp amount to 1.5 wt%, e.g. no greater than 1.5 wt %.

Potassium oxide, like sodium oxide, is a flux aiding in the melting ofthe batch materials by lowering the glass viscosity. A tramp amount ofK₂O in the glass is equal to or less than 0.07 wt %. In the practice ofthe invention, nonlimiting embodiments of the invention include, but arenot limited to, K₂O in a range selected from the group of 0 wt % or atramp amount to 0.5 wt %, e.g. no greater than 0.5 wt %; 0 wt % or atrace amount to 0.35 wt %, e.g. no greater than 0.35 wt %, and 0 wt % ora trace amount to 0.3 wt %, e.g. no greater than 0.3 wt %.

A reduction of the wt % of K₂O in the glass should be compensated for byan increase in the wt % of Na₂O to maintain the viscosity level of theglass. It is the usual practice to refer the sum of the wt % of Na₂O andK₂O as “R₂O”, i.e. R₂O equals Na₂O+K₂O. In the practice of theinvention, nonlimiting embodiments of the invention include, but are notlimited to, R₂O in a range selected from the group of 0 to 3 wt %, 0 to2.25 wt %, and 0.15 to 1.75 wt %.

Other ingredients can intentionally be added to the batch materials toalter the properties of the glass, e.g. but not limiting the inventionthereto, the color and solar absorption properties, or can be present inthe batch materials as impurities. For example, but not limiting theinvention thereto, Sample 5 discussed below had a tramp amount of 0.02wt % MnO₂. The following is a nonlimiting discussion of ingredients thatcan be added to the glass system of the invention or can be present asimpurities. As can be appreciated by those skilled in the art of makingglass, and in particular making flat glass, ingredients other than thoseingredients discussed below can be added to the glass system of theinvention or can be present as impurities or tramp ingredients.

In a nonlimiting embodiment of the invention, TiO₂ is optionally addedto the batch materials to lower the log 2 melting viscosity of theglass. TiO₂ in amounts below 0.05 wt % in the glass are considered animpurity or a tramp ingredient. TiO₂ in amounts up to 1 wt % are usuallyadded to the batch materials to provide a glass having desiredproperties, e.g. solar control property. Amounts greater than 1 wt % canalso be used to reduce ultraviolet transmission, e.g. see U.S. Pat. No.5,593,929; however, TiO₂ above 1 wt % in the glass can react with thetin bath as the bottom surface of the glass moves on the tin baththrough the forming chamber during the float glass process. The reactionturns the surface of the glass ribbon floating on the tin bath brown incolor. As can be appreciated, TiO₂ above 1 wt %, e.g. up to 5 wt %, canbe added to the batch materials, e.g. when glass color is not a requiredglass property, or when glass having a brown color is acceptable, orwhen the glass manufacturing process includes removing, e.g. grindingmaterial from a surface of the glass, or when the glass is made by asheet drawing process, e.g. the sheet drawing process developed by PPGIndustries, Inc. and known in the trade under the registered trademarkPENNVERNON, or the glass is made by the down draw or fusion sheetprocess. TiO₂ in amounts greater than 5 wt % are preferably not added tothe batch materials because the liquidus temperature of the glassincreases, which adversely affects the glass forming process. In thepractice of the invention, nonlimiting embodiments of the inventioninclude, but are not limited to, TiO₂ in a range selected from the groupof 0 wt % or trace amount to 5 wt %, e.g. no greater than 5 wt %; 0 wt %or trace amount to 2 wt %, e.g. no greater than 2 wt %; 0 wt % or traceamount to 1 wt %, e.g. no greater than 1 wt %; 0 wt % or trace amountsto 0.75 wt %, e.g. no greater than 0.75 wt %, and 0 wt % or tramp amountto 0.5 wt %, e.g. no greater than 0.5 wt %.

SO₃ can be added to the batch materials as a fining agent. SO₃ isusually added to the batch materials as part of another batchingredient, e.g. CaSO₄.2H₂O, usually added as gypsum, and/or Na₂SO₄,usually added as salt cake. SO₃ acts as a melting aid by preventingsilica scum (sand grain agglomeration) formation. In a nonlimitingembodiment of the invention, when SO₃ is used for chemical refining,usually about 0.2 to 0.4 wt % of SO₃ is added to the batch directly orindirectly by way of other batch materials containing SO₃. Not all theSO₃ in the batch is retained in the glass melt or the glass. Innonlimiting embodiments of the invention, SO₃ in a range selected fromthe group of 0.02 to 0.2 wt %; 0.02 to 0.15 wt %, and 0.02 to 0.1 wt %is retained in the glass.

The practice of the invention contemplates chemically refining themolten glass using SO₃ and physically refining the molten glass, e.g.using a vacuum chamber as taught in U.S. Pat. No. 4,919,697, whichpatent is hereby incorporated by reference. When the glass is vacuumrefined, the SO₃ present in the glass can be below detectable limits,e.g. below 10 parts per million (“ppm”).

Fe₂O₃ is added to the batch as a coloring agent and as an aid to therefining of molten glass by establishing temperature gradients thatprovide thermal convictions in the molten glass. If the Fe₂O₃ is onlyadded to the glass to maintain proper temperature gradients, it may beomitted when mechanical facilities, e.g. stirrers, are used tohomogenize the molten glass. Total iron in amounts below 0.05 wt % inthe glass is considered an impurity, or tramp material or ingredient. Ascan be appreciated by those skilled in the art, the total iron contentof the glass compositions disclosed herein is expressed in terms ofFe₂O₃ in accordance with standard analytical practice, regardless of theform of iron actually present. Likewise, the amount of iron in theferrous state is reported as FeO, even though it may not actually bepresent in the glass as FeO, The term “total iron” means total ironexpressed in terms of Fe₂O₃, and the term “FeO” means iron in theferrous state expressed in terms of FeO. In the practice of theinvention, nonlimiting embodiments of the invention include, but are notlimited to, total iron in a range selected from the group of 0 wt % or atramp amount to 1.25 wt %, e.g. no greater than 1.25 wt %; 0 wt % or atramp amount to 1 wt %, e.g. no greater than 1 wt %, 0 wt % or a trampamount to 0.5 wt %, e.g. no greater than 0.5 wt %, and 0 wt % or a trampamount to 0.15 wt %, e.g. no greater than 0.15 wt %.

Besides the above mentioned ingredients, batch melting and/or refiningagents, such as, but not limiting to the invention, NaCl, NaNO₃, KNO₃,BaSO₄, fluorides and combinations thereof can be added to the glassbatch and/or glass melt. Agents to alter the properties of the glass,e.g. solar control properties and color, e.g. compounds containingcobalt, nickel, cerium, neodymium, erbium, chromium, copper, manganese,molybdenum, tungsten, lanthanum, gold, silver, selenium and combinationsthereof can also be added to the glass batch and/or glass melt. As canbe appreciated, the additions to the glass batch and/or glass meltshould be concentrations in amounts not to impair the ion exchangeproperties of the glass.

As can be appreciated, the glass compositions of the invention can beproduced from conventional glass-making materials properly compoundedand thoroughly mixed so as to yield, when reacted, glasses of thedesired composition. Suitable batch materials include, but are notlimited to glass cullet, sand, soda ash (sodium carbonate), caustic soda(sodium hydroxide), magnesite, dolomite, talc, aluminum hydrate,feldspar, aplite, nepheline syenite, zircon sand, carbon, spodumene,lithium carbonate, and petalite.

The nonlimiting glass compositions of the invention have hightemperature properties, e.g. a log 2 temperature in the range of 2732°F. to 3272° F. (1500° C. to 1800° C.), and do not lend themselves tomelting, fining, homogenizing and forming in conventional float glassequipment, e.g., conventional overhead cross fire or end firingregenerative furnaces of the type used to melt soda-lime silicate glassbatch materials and often referred to as Siemens furnaces. Furnaces ofthis type can be used for melting the glass batch of the invention,fining the melted glass batch and homogenizing the fined glass by makingalterations to the firing equipment, e.g. but not limiting to theinvention, using gas-oxygen fired equipment in the furnace and/orproviding the furnace with electric boost, and/or lining the furnacewalls with high temperature refractory. The molten glass composition ofthe invention can be formed in a float bath by lining the walls of theforming chamber with high temperature refractory. Other types offurnaces and processes that can be used in the practice of the inventionare disclosed in U.S. Pat. Nos. 4,375,235 and 4,632,687, and in thearticle titled “Advances in the Process of Floating Borosilicate Glassand Some Recent Applications for Borosilicate Glass” by T. Kloss et al.published in the Journal of Glass Technology, Vol. 4, No. 6, December2000, pages 177 to 181, which patents and publication is herebyincorporated by reference.

Nonlimiting Samples 1-10 incorporating features of the invention weremade in the following manner. Batches, approximately 3000 grams performulation, were prepared using commercial ingredients. Unlessindicated otherwise, any glass samples made and discussed herein usedcommercial glass batch materials having known impurities or trampmaterials. The impurities and the tramp materials were included in thecalculation of the oxides in the glass samples. The weight of eachingredient was measured and used to determine the ingredients and wt %of the oxide in the glass. It was recognized that the volatilization ofthe materials could occur; however an assumption was made thatvolatilization would be at a minimum, and the calculations of the wt %of the oxides in the glass samples were made based on that assumption.

The batch for each sample was thoroughly mixed and about 1500 grams werecharged into a platinum crucible and placed in an electric furnaceheated to 2550° F. (1399° C.). After one hour, the remainder of thebatch, about 1500 grams, was added to the crucible and the temperatureincreased to 2800° F. (1538° C.). The crucible remained in the furnacefor an hour at 2800° F. (1538° C.); thereafter, the furnace temperaturewas increased to 2925° F. (1607° C.). After one hour, the glass melt(estimated to be at the log 3 temperature or 1000 poises) was removedfrom the furnace and poured into water at a temperature of 50° F. (10°C.). The solidified glass was dried, crushed, returned to the crucibleand placed in the furnace at the temperature of 2925° F. (1607° C.).After one hour the glass melt was once again removed from the furnaceand poured into cold water; the dried solidified glass was crushed andreturned to the crucible and placed in the furnace at the temperature of2925° F. (1607° C.). The temperature of the furnace was reduced to 2750°F. (1510° C.) and the crucible having the glass melt was left in thefurnace overnight (approximately 15 to 16 hours). The next morning thefurnace temperature was increased to 2925° F. (1607° C.). After onehour, the crucible was removed from the furnace and the molten glasspoured onto a steel plate heated to 500° F. (260° C.) and the moltenglass rolled with a steel cylinder. The glass slab was moved into afurnace heated to 1200° F. (649° C.) for 1 hour. The heating elements ofthe furnace were shut off, and the glass was left in the furnace to coolovernight to anneal the glass slab.

The compositions of Samples 1-10 are shown on Table 1 below. Asdiscussed above, the wt % of the oxides of Samples 1-10 were calculatedfrom the weight of the batch materials.

TABLE 1 Glass Composition of Laboratory Samples SAMPLE NO. Oxide 1 2 3 45 6 7 8 9 10 SiO₂ 66.68 66.78 66.78 67.78 69.29 64.83 65.23 68.3 68.1766.67 Al₂O₃ 22 22 22 21 23.51 22.5 21.97 22 22 22 Li₂O 5.5 5.5 5.5 5.56.50 5.53 4.94 5.5 5.5 5.5 Na₂O 1 2 1.5 1.5 0.39 1.27 0.99 0.7 1 1 K₂O0.07 0.07 0.07 0.07 0.1 0.07 0.07 0.15 0.15 0.15 CaO — — — — 0.04 0.051.27 — — — MgO 2.2 1.6 1.6 1.60 — 2.49 0.89 1.5 1.5 1.5 SO₃ — — — — 0.090.09 0.19 — — — Fe₂O₃ 0.05 0.05 0.05 0.05 0.061 0.05 0.05 0.05 0.05 0.05MnO₂ — — — — 0.02 0.01 0.01 — — — ZrO₂ — — — — — — 1.19 — — 1.5 SnO₂ 0.50.5 0.5 0.5 — 0.5 0.27 0.47 0.43 0.43 TiO₂ — — — — 0.01 0.01 1.89 — — —P₂O₅ 1 1 1 1 — 1.05 1.06 0.76 0.68 0.68 ZnO — — 0.5 0.5 — — — 0.57 0.510.51 B₂O₃ 1 0.5 0.5 0.5 — 1.55 — — — — BaO — — — — — — — 0.002 0.0050.01 Total (calc) 100 100 100 100 100 100 100.02* 100 100 100 *The totalwt % for Sample 7 is greater than 100% due to a round-off error.

The surfaces of the glass slab of Samples 1-7 were viewed under 50× to1000× magnification. The surface of the glass slab contacted by thesteel roller (hereinafter also referred to as the “upper surface”) hadspaced clusters of massive amounts of crystallites having a size ofabout 15 to 50 microns. The clusters were randomly spaced from oneanother on the upper surface. The surface of the glass slab contactingthe steel plate (hereinafter also referred to as the “lower surface”)had a few spaced clusters of crystallites. The clusters on the lowersurface had significantly fewer number of crystallites in the clustersthan the upper surface of the slab. It was observed that Sample 5 didnot have any clusters on the lower surface. Although the crystallites inthe clusters of Samples 1-7 were not counted, visually it appeared thatthe clusters in the upper surface had 2 orders of magnitude morecrystallites than the clusters of the lower surface. Approximately 0.03inch (0.76 millimeters (“mm”)) of the upper surface was removed using aabrasive slurry and the surface polished. The polished upper surface wasviewed under 50× to 1000× magnification. The polished upper surface andthe lower surface of the glass slab had approximately the same number ofcrystallites in the spaced clusters. An X-Ray diffraction of thecrystallites was made and the results showed that the crystallites werephases of different crystals (LiAl(SiO₃)₂ and LiAlSi₃O₈) of lithiumaluminum silicate and spodumene (LiAlSi₂O₆). The glass slabs of Samples8-10 crystallized as they cooled to room temperature, and thecrystallization resulted in the glass slab fracturing into fragments. Asa result the properties of these samples could not be measured. Thecause(s) of the crystallites observed in Samples 1-7, and thecrystallization observed in Samples 8-10, has (have) not beendetermined. Mechanisms that are known to cause crystallites andcrystallization include incomplete melting of, and impurities in, thebatch materials that nucleate crystals, and phase separation.

As mentioned above, one of the mechanisms that can cause crystallites,e.g. the crystallites of the type observed in Samples 1-7, is incompletemelting of the batch ingredients. To determine if the cause of thecrystallites is incomplete batch melting, Samples 11-16 having thecompositions shown on Table 2 below were made.

TABLE 2 Glass Composition of Laboratory Samples Sample No. CommercialCommercial Oxide 11 12 13 14 15 16 17 18 19 20 Product A Product B SiO₂69.29 64.29 74.29 68.54 63.54 73.54 66.29 63.42 63.42 63.42 61.38 70.54Al₂O₃ 23.51 28.51 18.51 23.26 28.26 18.26 26 24.87 24.87 24.87 18.0219.25 Li₂O 6.5 6.5 6.5 7.5 7.5 7.5 7 7 7 7 5.3 4.14 Na₂O 0.39 0.39 0.390.39 0.39 0.39 0.39 2.39 2.39 0.39 10.79 0.26 K₂O 0.1 0.1 0.1 0.1 0.10.1 0.1 0.1 0.1 0.1 0.11 0.23 CaO 0.04 0.04 0.04 0.04 0.04 0.04 0.040.04 0.04 0.04 0.11 — MgO — — — — — — — — 2.6 2 0.05 — SO₃ 0.09 0.090.09 0.09 0.09 0.09 0.09 0.09 0.09 0.09 0.08 — Fe₂O₃ 0.061 0.061 0.0610.061 0.061 0.061 0.061 0.061 0.061 0.061 0.118 0.05 MnO₂ 0.02 0.02 0.020.02 0.02 0.02 0.02 0.02 0.02 0.02 — — ZrO₂ — — — — — — — — — — 4.021.47 SnO₂ — — — — — — — — — — — — TiO₂ 0.01 0.01 0.01 0.01 0.01 0.010.01 0.01 0.01 0.01 0.02 1.78 P₂O₅ — — — — — — — — — — — — ZnO — — — — —— — — — — — 1.66 B₂O₃ — — — — — — — 2.00 — 2.00 — — Sb₂O₃ — — — — — — —— — — — 0.32 F⁻ — — — — — — — — — — — 0.26 Total 100 100 100 100 100 100100 100 100 100 99.96* 99.91* *The total wt % for Commercial Products Aand B are less 100% due to a round-off error.

The following procedure was practiced to make Samples 11-16. One hundred(100) grams of batch for each sample was thoroughly mixed and chargedinto a platinum crucible and placed in an electric furnace heated to3000° F. (1649° C.), After two and one half hours, the glass melt(estimated to be at the log 3 temperature or 1000 poises) was removedfrom the furnace and poured into water at a temperature of 50° F. (10°C.). The solidified glass was dried, crushed, returned to the crucibleand placed in the furnace at the temperature of 3000° F. (1649° C.).After two hours the glass melt was once again removed from the furnaceand poured into cold water; the dried solidified glass was crushed andreturned to the crucible and placed in the furnace at the temperature of3000° F. (1649° C.). The temperature of the furnace was reduced to 2750°F. (1510° C.) and the crucible having the glass melt was left in thefurnace overnight (approximately 15 to 16 hours). The next morning thefurnace temperature was increased to 3000° F. (1649° C.). After twohours the crucible was removed from the furnace and the molten glasspoured onto a steel plate and formed a glass slab as it cooled. Theglass slab was moved into a furnace heated to 1350° F. (732° C.) for 1hour. The furnace was shut off, and the glass slab was left in thefurnace to cool overnight to anneal the glass slab.

Sample 11 has a glass composition similar to the glass composition ofSample 5. Sample 5 was duplicated because it had fewer crystallites and,as shown and discussed below, had better chemical strength results thanSamples 1-4, 6 and 7, as will be discussed later in more detail. Theglass slabs for Samples 13, 14 and 16 were opaque. The upper surface ofthe glass slabs of Samples 11, 12 and 15 were ground as previouslydiscussed washed, dried and observed under 50× to 1000× magnification.Samples 11, 12 and 15 had a crystallite count similar to the crystallitecount of Sample 5. Based on the results of the experiment, it wasconcluded that the crystallites are not caused by unmelted batch.

Samples 17-20 having the compositions shown on Table 2 were made toobserve the effect on crystallization by varying the wt % of, or adding,SiO₂, Al₂O₃, Li₂O, Na₂O, MgO and B₂O₃ as compared to wt % ingredientsfor Sample 5. Samples 17-20 were made using the procedure to makeSamples 1-7. Glass pieces of Sample 17 had a crystallite count similarto Sample 5. Comparing the wt % of ingredients of Samples 5 and 17, theSiO₂ was decreased and the Al₂O₃ and Li₂O were increased. Based on atwo-sample comparison, there is an indication that decreasing wt % ofSiO₂ and Li₂O and increasing the wt % of Al₂O₃ has minimal, if any,effect on crystallization. Comparing the wt % of ingredients of Sample18 to Sample 5, Sample 18 had a decrease in SiO₂; an increase in Al₂O₃,Li₂O and Na₂O, and the addition of B₂O₃. The glass pieces of Sample 18had significantly more crystallites than Sample 5, Comparing the wt % ofingredients of Sample 19 to Samples 5 and 18, Sample 19 was similar toSample 18 except Sample 19 had no B₂O₃ and had the addition of MgO. Theglass pieces of Samples 18 and 19 had significantly more crystallitesthan Sample 5. Based on a three sample comparison, it appears thatsignificant decrease in wt % of SiO₂, e.g. a 10% decrease, significantincrease in wt % of Na₂O, e.g. 600% increase, and moderate increase inwt % of Al₂O₃, e.g. a 5% increase, compared to wt % of ingredients inSample 5 increases crystallization. The addition of MgO and B₂O₃ on atwo-sample comparison, e.g. Samples 18 and 19 is inconclusive, i.e.either has no effect on crystallization or a significant effect oncrystallization. No data regarding crystallization for Sample 20 wascollected because Sample 20 crystallized at 1300° F. (704° C.), which isbelow the desired glass bending temperature.

Also shown on Table 2 is Commercial Products A and B. Commercial ProductA is a glass made on Jan. 25, 1997, at the Carlisle Plant of PPGIndustries, Inc. located in Carlisle, Pa. Commercial Product B is aglass ceramic made in 1973 and sold by PPG Industries under theregistered trademark Hercuvit. Commercial Products A and B are shown onTable 2 to compare glass compositions and properties of the glassesincorporating nonlimiting embodiments of the invention to glasscompositions and selected properties of previously available commercialproducts.

The following experiment was conducted to determine the effect, if anyheat has on the crystallites. Glass pieces cut from the glass slabs ofSamples 1-7, 17 and 20 were thermally treated. The thermally heatingprocess included the steps of placing the pieces in a Lindberg furnace,ramping the furnace up to the desired temperature, leaving the pieces inthe furnace for a desired time period, turning the heat source off,partially opening the furnace door and allowing the glass pieces tocool. The temperature of the glass pieces was determined by reading theoutput of a thermocouple having a zirconium wire positioned on thesurface of the glass piece. After the pieces reached room temperature,the major surfaces of the pieces were viewed under 50× to 1000×magnification. Unless indicated otherwise below, new glass pieces wereused for each heating operation.

Glass pieces of Samples 1-4 and 6 were heated to a temperature of 900°F. (482° C.) for a period of 4 hours. No change in the number or size ofthe crystallites was observed. Glass pieces of Samples 1 and 6 wereheated at 1100° F. (593° C.) for 4 hours; no change in the number orsize of the crystallites was observed. Glass pieces of Samples 1-4 and 6were heated to a temperature of 1300° F. (704° C.) for a period of 1hour. No change in the number or size of the crystallites was observedfor Samples 1-4; however the glass piece for Sample 6 showed a slightincrease in the size of the crystallite. Glass pieces of Samples 1-4 and6 were heated to a temperature of 1300° F. (704° C.) for a period of 2hours. There was a slight increase in the size of the crystallites inSamples 1 and 2 compared to the increase in the size of crystallites ofSample 6; a moderate increase in the size of the crystallites forSamples 3 and 4 compared to the increase in the size of the crystallitesof Samples 1, 2 and 6, and a large increase in the size of thecrystallites of Sample 6 compared to the size of the crystallites forSamples 1 and 2.

Glass pieces of Samples 1-4, 6 and 7 were heated to a temperature of1300° F. (704° C.) for a period of 4 hours. The following was observedfor the glass pieces of Samples 1-4, 6 and 7. Sample 1: the crystallitesize doubled, there was an increase in the number of surfacecrystallites and there were crystallites in the area of the zirconiumthermocouple wire; Samples 2-4: the crystallite size doubled, and therewere crystallites in the area of the zirconium thermocouple wire; Sample6: the piece was white and polycrystalline and had a few surfacecrystallites, and Sample 7: there was a slight increase in the number ofcrystallites. Glass pieces of Samples 5, 17 and 20 were heated to atemperature of 1300° F. (704° C.) for a period of 6.2 hours. No changesin the number or size of the crystallites for Samples 5 and 17 wereobserved. The glass piece of Sample 20 had a hazy appearance indicatingan increase in the size or number of crystallites. Glass pieces ofSamples 5, 17 and 20 were heated to a temperature of 1427° F. (775° C.)for a period of 1 hour. No change was observed in the number or size ofthe crystallites of Samples 5 and 17; however Sample 17 had crystallitesat the position of the zirconium thermocouple wire. The glass piece ofSample 20 was hazy and full of crystallites.

Glass pieces of Samples 1, 5-7, 17 and 20 were heated to a temperatureof 1600° F. (871° C.) for a period of 1 hour. The glass piece of Samples1 and 6 had a beige color; the glass piece of Sample 7 had a gray color;the glass of Sample 20 had a white color, and the glass pieces ofSamples 1, 6, 7 and 20 were nearly completely crystallized. The glasspieces of Samples 5 and 17 were hazy and had an increase in the size ofthe crystallites. Glass pieces of Samples 1, 5-7, 17 and 20 were heatedto a temperature of 1800° F. (982° C.) for a period of 1 hour, The glasspiece of Samples 1 and 6 had a beige color; the glass piece of Sample 7had a gray color; the glass pieces of Samples 5, 17 and 20 had a whitecolor, and the glass pieces of Samples 1, 5-6, 7, 17 and 20 were nearlycompletely crystallized. Glass pieces of Sample 5 were heated to atemperature of 2051° F. (1122° C.) for a period of 3 minutes and at atemperature of 2054° F. (982° C.) for a period of 22.7 minutes. Theglass pieces of Sample 5 had a white color and were nearly completelycrystallized.

The above experiments were performed on glasses that were melted andannealed at high temperatures in laboratory crucibles indicating it ispossible to form these glasses without significant crystallization. Theappearance of crystals predominantly on the top surface indicates thatthe occurrence of these crystals is at least partially dependent uponprocess conditions evident at the glass surface and not solely due toglass composition. The experiments further indicate thatpost-glass-forming heat exposures can cause widespread crystallizationof the glass, The temperature required for this crystallization isgenerally higher than the expected bending temperature of the glass,which should allow the glass to be shaped in a secondary bendingoperation without significant crystallization.

From the above experiment, it is concluded that the glass composition isimportant in determining how much crystallization occurs during heattreatment at high temperatures.

Glass pieces of each one of the samples was not heat treated at each ofthe times and temperatures because heat treatment of glass pieces of allthe samples at all of the times and temperatures discussed above is notexpected to provide any additional insight as to the effect of time andtemperature on the crystallites.

As previously mentioned, Table 2 lists the composition of CommercialProducts A and B. The wt % of the ingredients of Commercial Product A,except for Li₂O, was determined using X-ray fluorescence spectroscopy,and the wt % of Li₂O was measured using atomic absorption spectroscopy.The file describing the procedure to determine the ingredients, and thewt % of the ingredients, of Commercial Product B is not available;however it is believed that the wt % of all the ingredients, exceptLi₂O, were determined using X-ray fluorescence spectroscopy. It isfurther believed that Li₂O was determined using atomic absorptionspectroscopy.

Coupons were cut from the slabs of Samples 1-7 and Commercial Product Ato determine thermal and strength properties of the glass compositions.In general, the glass slabs, or pieces of the slab, of Samples 1-7 andCommercial Product A, were polished to the desired thickness by usinggrit mixed with water. Coupons of various sizes were cut from thepolished slabs. Grinding and polishing of the coupons was performed asneeded to provide a smooth surface. The edges of the coupons wereslightly rounded using a 220 grit belt sander.

The log 10 viscosity temperature (bending temperature) and liquidustemperature of Sample 1 and Commercial Product A were measured. The log10 temperature was measure according to ASTM C1350M-96, standard testmethod for measurement of viscosity of glass between softening point andannealing range by beam bending (Annual Book of ASTM Standards, Vol.15.02 (2001) 423-426). The coupon had a length of 3.5 centimeters(“cm”), a thickness of about 3.1 millimeters (“mm”) and a width of 2.8mm. The temperature during the test was raised 5° C. per minute andlowered 4° C. per minute. The procedure was repeated three additionaltimes and the four measurements averaged. The liquidus temperature wasmeasured in accordance with the procedure set out in ASTM methodC829-81.

Sample 1 had an average log 10 viscosity temperature of 1414° F. (768°C.) and a liquidus temperature of 2437° F. (1336° C.). Sample 5 had anaverage log 10 viscosity temperature of 1490° F. (810° C.), a liquidustemperature of 2575° F. (1413° C.) and a softening point temperature(log 7.6) of 1724° F. (940° C.); Sample 17 had an average log 10viscosity temperature of 1499° F. (815° C.), a liquidus temperature of2575° F. (1413° C.) and a softening point temperature of 1720° F. (938°C.), and Sample 20 had an average log 10 viscosity temperature of 1328°F. (720° C.), a liquidus temperature of 2493° F. (1367° C.), and asoftening point temperature of 1544° F. (840° C.). The CommercialProduct A had a log 10 viscosity temperature of 1112° F. (600° C.) and aliquidus temperature of 1533° F. (834° C.). The historical recordedsoftening point temperature (log 7.6) for Commercial Sample B is 1765°F. (963° C.) and the historical recorded liquidus temperature is 2430°F. (1332° C.). The above results show the glass compositions of Samples1, 5, 17 and 20 have a log 10 viscosity temperature and liquidustemperature higher than Commercial Product A, and Samples 5, 17 and 20had a softening point temperature lower than the softening pointtemperature of Commercial Product B, and a liquidus temperature aboutequal to or slightly higher than Commercial Product B. From the aboveresults, it is noted that the glass of Samples 1, 5, 17 and 20 willmaintain their shape at significantly higher temperatures thanCommercial Product A. This makes the glass of the invention suitable foruse in heated environments.

As can be appreciated from the above measured temperatures, nonlimitingembodiments of the invention have a log 10 viscosity of greater than1328° F. (720° C.), greater than 1414° F. (768° C.) and in thetemperature range of 1328° F. (720° C.) to 1499° F. (815° C.); aliquidus temperature greater than 2437° F. (1336° C.), greater than2493° F. (1367° C.), and in the temperature range of 2437° F. (1336° C.)to 2575° F. (1413° C.), and a log 7.6 softening point temperaturegreater than 1544° F. (840° C.), greater than 1720° F. (938° C.), and inthe temperature range of 1544° F. (840° C.) to 1724° F. (940° C.). Theinvention is not limited to the temperature ranges discussed hereinaboveand nonlimiting embodiments of the invention contemplate glasses havinga log 10 viscosity temperature greater than 1280° F. (694° C.), greaterthan 1350° F. (732° C.) and greater than 1414° F. (768° C.), and aliquidus temperature greater than 2350° F. (1288° C.), greater than2400° F. (1316° C.) and greater than 2436° F. (1336° C.).

Glass coupons of Samples 1-7 and Commercial Product A were chemicallytempered under varying conditions of time and temperature in a moltenbath of 100% pure grade sodium nitrate. As can be appreciated, thecomposition of the molten bath of sodium nitrate is not limiting to theinvention and other bath solutions that contain (or partially contain)other sodium salts such as sodium sulfate and sodium chloride are wellknown in the art. Optionally, the bath can be a mixture of sodium andpotassium salts, although the portion of sodium salts preferably ishigh, e.g. above 50%, to obtain the acceptable abraded strength in thetempered glass article, e.g. above 40 KPSI (276 mPa). Sodium nitratebaths for chemical strengthening glass are well known in the art and theinvention is not limited thereto. Based on the foregoing, no furtherdiscussion is deemed necessary.

The glass coupons of Samples 1-7 and Commercial Product A were weighedand placed in slots of a basket and submerged in the sodium nitrate bathfor a predetermined time period with the bath at a preset temperature.Shown on Table 3 is data representative of the time period in hours andtemperatures in ° C. for coupons for each Sample and the CommercialProduct A. At the completion of the immersion, the samples were removedfrom the bath, allowed to cool in ambient air, rinsed with water toremove solidified salts from the glass surfaces, and the coupons dried.

The glass coupons were weighed after chemical tempering to determineweight gain of the coupons. The area of each surface of the weighedcoupons was measured and the area of the six surfaces added and the sumdivided into the weight gain for the coupon to give a weight gain perarea. The weight gain of the coupons provides a quantitative measure ofthe degree of ion exchange. The average of the weight gain per area forthe coupons and the Commercial Product A is listed on Table 3.

TABLE 3 Chemical strengthening particulars of Samples 1-7 coupons andthe Commercial Product are listed on the Table 3. Center Tension Center(KPSI) Tension Time Weight Case (case (KPSI) Sample in Temp. Gain DepthAMOR MOR depth (0.35 × 3 in. No. Hours ° C. (mg/cm²) (mils) (KPSI)(KPSI) coupon) coupon) 1 16 374 1.4   7/10.8^(B) — — 6.1 6.5 1 22 3741.6 10.6/12.8^(B) 76 89 6.4 7 1 16 405 2.0 12.7/15.2^(B) 78 108 8  9/9.6^(C) 1 22 405 2.4   14/17.4^(B) 77 112 8.7 10.4 2 16 374 — 10.4 —— 6.7 7.4 2 22 374 1.9 12.7 72 91 6.5 8.6 2 16 405 2.3 14.9 77 93 9.010.4/10.9^(C) 2 22 405 2.7 16.5 75 82 11 11.9 3 16 374 — 9.7 — — 6.2 7 322 374 1.8 11.3 75 90 7 7.8 3 16 405 2.2 14.1 78 90 8.3  9.8/10.3^(C) 322 405 2.6 16 77 102 10 11.7 4 16 374 — 10.1 — — 5.5 7 4 22 374 1.8 12.174 76 5.7 8.3 4 16 405 2.2 14.2 67 78 8.3 10.2/10.5^(C) 4 22 405 2.615.7 73 71 10.1 12 5 8 374 1.4 9.7 43 — — 7.4 5 16 374 2.5 13.3 — — —10.3 5 8 405 2.5 14.8 — — — 12.6 5 18 405 3.6 16.5 — — — 14.9 6 16 3741.4 8.1 — — — 5.7 6 22 374 1.5 10.4 — — — 6.7 6 16 405 1.8 12.1 — — 7.6— 6 22 405 2.2 12.7 — — — — 7 22 374 1.6 10.7 78.6 106 — 6.9 to7.7/8.3^(C) 7 16 385 1.4 10.2 — — — 7.3 7 22 385 1.6 11.1 — — — 8.4 7 16405 2.0 11.5 78.3 99 — 8.4 to 8.6/10^(C) 7 22 405 2.3 13.3 79.8 107 —9.7 to 10.1/11.7^(C) 7 46 425 3.3 14.7 74.7 93 — 15.1/16.6^(C) 7 16 4252.4 15.1 — — — 11.2 7 22 425 2.7 16.8 — — — 12.6 CP-A 22 385 1.0 15.6 —— — 2.7 CP-A 16 405 1.1 13.7 — 66 — 2.8 CP-A 22 405 1.3 15.2 — 72 — 3.1CP-A 46 405 1.9 20.3 — 66 — 4.1 CP-A 16 425 1.4 15.3 — — — 2.5 CP-A 22425 1.6 15.8/18.3^(B) — 62 — 2.8/3^(C)   CTA HV 1000 HK 1000 CTA 0.35 ×3 Vickers Knoop Time CD inches hardness hardness Sample in Temp. couponcoupon (kg/mm²) (kg/mm² No. Hours ° C. (lbs./in.) (lbs./in.) (Note G)(Note G) 1 16 374 678 700 614 (576) 520 (503) 1 22 374 600 718 604 (576)513 (503) 1 16 405 749 886/919^(C) 598 (576) 522 (503) 1 22 405 733 977586 (576) 505 (503) 2 16 374 684 754 606 (580) 529 (499) 2 22 374 554831 600 (580) 503 (499) 2 16 405 813  961/1006^(C) 600 (580) 510 (499) 222 405 979 1072 597 (580) 508 (499) 3 16 374 656 730 611 (584) 526 (503)3 22 374 680 790 606 (584) 509 (503) 3 16 405 743 944/984^(C) 599 (584)505 (503) 3 22 405 910 1080 600 (584) 519 (503) 4 16 374 688 697 606(577) 520 (495) 4 22 374 632 804 601 (577) 524 (495) 4 16 405 768922/959^(C) 606 (577) 505 (495) 4 22 405 947 1045 590 (577) 506 (495) 58 374 — 789 — — 5 16 374 — 1033 — — 5 8 405 — 1176 — — 5 18 405 — 1390 —— 6 16 374 — 632 — — 6 22 374 — 706 — — 6 16 405 769 — — — 6 22 405 — —— — 7 22 374 — 689 to 773 — — 7 16 385 — 742 — — 7 22 385 — 839 — — 7 16405 — 822/828^(H) — — 7 22 405 — 895/951^(H) — — 7 46 405 — 1269 — — 716 425 — 1012 — — 7 22 425 — 1123 — — CP-A 22 374 — — — — CP-A 22 385 —— — — CP-A 16 405 — — — — CP-A 22 405 — — — — CP-A 46 405 — — — — CP-A16 425 — — — — CP-A 22 425 — — — — NOTES: A. CP-A is designation forCommercial Product A ^(B)Values on the right are for coupons havingdimensions of 0.22 inches (‘in.”) by 0.24 in. by 3 in. (5.6 millimeters(“mm”) by 6.1 mm by 76 mm). Otherwise, the values were measured onsamples about 0.11 in. by 0.11 in. by 1.5 in. (2.8 mm by 2.8 mm by 3.8centimeters (“cm”)). ^(C)Numbers on the left are on coupons having acontact area of 0.35 in. × 3 in. (0.89 mm by 7.6 cm), and the numbers onthe right have a contact surface of 1 in. × 8 in. (2.54 cm by 20 cm). D.Numbers indicate distribution range of measured values. E. Numbers thatare connected by a “to” indicate distribution range of measured values.F. The size of the coupon was 1 in. × 8 in. (2.54 cm by 20 cm). G.Numbers in parenthesis are test results for coupons that were notchemically tempered. ^(H)The two values are two different measurementsof the same coupons. A cell having a “—” indicates that no test resultsare available.

A comparison of the weight gain of the coupons of Samples 1-7 andCommercial Product A for similar times and temperatures shows that thenonlimiting glass compositions of the invention represented by Samples1-7 have a higher weight gain than Commercial Product A. A higher weightgain is representative of a faster exchange rate and a greater degree ofion exchange. It may be indicative of higher stress levels and strengthif stress relaxation is insignificant. No historical weight gain datahas been identified for Commercial Product B.

The case depth for two coupons 3 mm by 3 mm by 3.8 cm for each of thedifferent times and temperatures for each Sample 1-7 were measured byviewing the cross section of the coupon with a quartz wedge microscope.The case depths of two coupons for Sample 1 having dimensions of 5.6 mmby 6.1 mm by 7.6 cm were measured for a comparison of case depths for athicker piece of glass. Commercial Product A coupons were 4.8 mm by 3.0mm by 3.8 cm. The results of the measurement are listed on Table 3. Forthe same time and temperature, the case depth of the thinner couponsappears to be equal to or less than the case depth for CommercialProduct A. However, the thicker coupons of Sample 1 have a greater casedepth. It is recognized in the art that thicker glasses have deeper casedepth; therefore, a comparison between case depth for thick glasscoupons and thinner glass coupons should not be made. Historical datafor Commercial Product B of 5.6 mm thickness show a case depth of 13.5mils for coupons heated at a temperature of 454° C. for 4 hours, and acase depth of 26 mils for coupons heated at the same temperature for 16hours. The case depth is a useful property to indicate resistance todamage and it is believed that case depths in excess of about 10 milsare desirable for certain applications of the glass such as aircrafttransparencies. The nonlimiting glass compositions of the invention meetthat criterion.

Coupons for modulus of rupture (“MOR”) measured 5.4 cm by 5.4 cm by 3.0mm thick, and coupons for abraded modulus of rupture (“AMOR”) measured2.54 cm by 20.3 cm by 3 mm thick. Eight (8) to 12 coupons for AMOR had2.54 cm diameter circle abraded in the center of a major surface of thecoupon with 2 cm³ of 100B Norton Alundum abrasive or equivalent at 30pounds per square inch (“psi”) for five seconds. A similar number ofcoupons without an abraded surface were set aside for the MOR testing.The test to measure AMOR uses a four point simply support arrangement,which includes placing a major surface of a coupon on parallel 6.2 mmdiameter cylinders spaced 15.2 cm apart and placing a platform on twoparallel 6.2 mm diameter cylinders spaced 5.08 cm from one another onthe upper surface of the coupon. For the AMOR test, the surface of thecoupon having the abraded portion faces the platform, and the abradedportion is between the cylinders supporting the platform. An increasingmeasured force is applied to the platform until the coupon fractures.

MOR coupons are tested using a concentric ring apparatus. In this test,the glass coupon is placed on the support ring of 2 in. (5.08 cm)diameter and the load ring of 1 in. (2.54 cm) diameter is placed on theglass so that the rings are concentric and the glass is centered betweenthe rings. Prior to assembly, the glass is inspected for obvious flaws,which are marked, and the sample is taped so as to enable adetermination of the location of the origin of the fracture. On rareoccasions, the glass fractured at a predetermined flaw or outside thehighly stressed area and, in these cases, the data was omitted fromconsideration.

The force required to fracture the coupon and the dimensions of thecoupon are used to calculate the stress at failure according to knownformulas. The average of the coupons for each of Samples 1-7 andCommercial Product A is listed on Table 3. The AMOR for CommercialProduct A was not measured using this test. The historical data forCommercial Product B shows an MOR of 60 KPSI and 50 KPSI after chemicalstrengthening for 4 and 16 hours, respective at 454° C. The AMOR and MORvalues for coupons of Samples 1-7 generally have higher values thanCommercial Product A and it is expected that the chemically temperedglass of the instant invention is as strong if not stronger than theglass of Commercial Product A.

A coupon of each Sample 1-7 used to measure case depth was used tomeasure the center tension. The center tension of a coupon of eachSample 1-6 used to measure case depth was used to measure the centertension. For Sample 7, the dimensions were 3.0 mm thick by 4.8 mm depthby 3.8 cm length. The center tension was measured using a quartz wedgemicroscope to obtain the optical retardation at the center of thecoupon. The optical retardation divided by the sample depth orbirefringence was converted to the tensile stress using thestress-optical coefficient of 1.84 (psi-inch)/mu. The use of thiscoefficient for Samples 1-7 allows a close approximation of centertension due to the relative similarities in glass composition. Thecenter tension values for the case depth coupons measured are listed inthe column titled “Center Tension (KPSI) (case depth coupon).” Centertension values have a very high dependence on thickness of the couponsince the surface of all coupons are nearly similarly compressed and thebalancing tensile forces on the interior of the glass are applied overthe central region which can vary widely in thickness.

The center tension of a coupon having a major surface of 0.35 in by 3 in(8.9 mm by 7.6 cm) or 1 in by 8 in (2.54 cm by 20 cm) for Samples 1-7and the Commercial Product A were additionally measured practicing theprocedure discussed above. The measurements are listed on Table 3. It isbelieved that these numbers provide a more accurate estimate of thecenter tension of a larger plate since the edge effects occur in asmaller portion of the depth of the sample. The number on the left forSamples 1-4 for 16 hours at 405° C. were the result of measurements on asurface 0.35 in by 3 in., and the number on the right for surface having1 in by 8 in. It can therefore be observed that, for samples of shortdepth, the reported values are an underestimate of the value in a largerplate. Small samples are used since the image of the stress profile asviewed through the quartz wedge microscope deteriorates as the samplebecomes too deep and the tensile stress becomes large.

The Center Tension Area (CTA) value provides a measure of the totalstress in the glass and partially takes into account the varyingthicknesses of samples. The CTA value is calculated from the samplethickness, case depth, and center tension values and is an attempt toestimate the stress through the thickness of the glass. Thus, the centertension value is multiplied by the (thickness of the coupon minus 2times the case depth of the coupon) to obtain the CTA value inpounds/inch. The data indicates that Samples 1-7 have a significantlyhigher CTA values than Commercial Product A.

Chemically tempered coupons and non-tempered coupons of Samples 1-7 weremeasured for Knoop hardness and Vickers hardness according to ASTMC-730-98 [2003] using a 1000 gram force to determine the resistance ofthe coupon to surface penetration. The results of the Vickers hardnesstest is in the column of Table 3 titled “HV 1000” and the results of theKnoop hardness test is the column titled “HK 1000.” The values are givenin kilograms force per square millimeter (kg-f/mm²). The values inparenthesis are the measurement for the non-tempered coupons. From Table3, it is seen that all the chemically tempered coupons were strongerthan the untempered coupons. No measurements are available forCommercial Products A and B.

The chemical durability of the glass of Samples 1-4 and 7 weredetermined by submerging glass pieces of a sample into a container ofboiling 0.50 wt % sulfuric acid and covering the container. After 30minutes, the glass pieces are removed from the acid, cooled, rinsed anddried. The weight in grams and area in square cm of the glass pieceswere measured before putting the glass pieces in the acid and after thesamples were dried. The difference in weight of the pieces is divided bythe area of the pieces and the quotient multiplied by 2 to obtain theloss in grams per square cm-hour. Below is the glass loss for Samples1-4 and 7 in milligrams per square cm-hour (“mg/cm²-hr”): The loss is inthe range of 0.002 to 0.014 mg/cm²-hr.

Sample Loss 1 0.013 2 0.014 3 0.005 4 0.002 7 0.007

Historical data for Commercial Product A shows a loss of 0.007milligrams per square cm-hour. Previous experience with various glassesindicates that values of about 0.02 milligrams per square cm-hour andless are suitable for use where the surface of the glass is exposed. Ascan be concluded from the above data, the acid resistances of theglasses of the instant invention are similar to that of CommercialProduct A. Commercial Product A has a commercial use history of beingquite durable. No historical data is available for Commercial Product B.The compositions of Samples 21-26 listed on Table 4 below wereformulated based on the ingredients and performance of Samples 1-20discussed above.

The compositions of Samples 21-26 listed on Table 4 below arehypothetical compositions formulated based on the ingredients andperformance of Samples 1-20 discussed above.

TABLE 4 Glass compositions of predictive examples. SAMPLE NO. Oxide 2122 23 24 25 26 SiO₂ 66.75 66.25 66.75 65.1 64.6 65.25 Al₂O₃ 22 22 22 2222 22 Li₂O 6 6.5 6 6 6.5 6 Na₂O 1.5 1.5 1.5 1.5 1.5 1.5 K₂O 0.1 0.1 0.10.25 0.25 0.1 CaO — — — 0.5 0.5 0.5 MgO 1.6 1.6 1.6 1.6 1.6 1.6 SO₃ — —— — — — Fe₂O₃ 0.05 0.05 0.05 0.05 0.05 0.05 MnO₂ — — — — — — ZrO₂ — —1.5 1 1 1.5 SnO₂ 0.5 0.5 0.5 0.5 0.5 0.5 TiO₂ — — — 0.5 0.5 0.5 P₂O₅ 1 1— — — — ZnO 0.5 0.5 — 0.5 0.5 — B₂O₃ — — — 0.5 0.5 0.5 Sb₂O₃ — — — — — —Total 100 100 100 100 100 100

The ingredients and wt % range for each ingredient of nonlimiting glasscompositions of the invention developed from Tables 1, 2 and 4 arelisted in Table 5 below. The first column lists the ingredients and thewt % range for glasses developed from the ingredients of Samples 1-7 and11-26 listed on Tables 1, 2 and 4. The second column lists theingredients and the wt % range for glasses developed from theingredients of Samples 1-7 and 17-20 listed on Tables 1 and 2. The thirdcolumn lists the ingredients and the wt % range for glasses developedfrom the ingredients of Samples 17-20 listed on Table 2, taking intoconsideration the operational ranges of ingredients based on the othersamples. The fourth column lists the ingredients and the wt % range forglasses developed from the ingredients of Samples 1-7 listed on Table 1.The fifth column lists the ingredients and the wt % range for glassesdeveloped from the ingredients of Samples 21-26 listed on Table 4.

TABLE 5 Column 1 Column 2 Column 3 Column 4 Column 5 Samples 1-7 and11-26 Samples 1-7 and 17-20 Samples 17-20 Samples 1-7 Samples 21-26Ingredient Percent by Weight Percent by Weight Percent by Weight Percentby Weight Percent by Weight SiO₂ 63.42 to 74.29 63.42 to 69.29 63.42 to66.29 64.83 to 69.29   64.6 to 66.75  Al₂O₃ 18.26 to 28.51 21 to 2624.87 to 26   21 to 23.51 22 Li₂O 4.94 to 7.5  4.94 to 7   5 to 7 4.94to 6.5    6 to 6.5 Na₂O 0.39 to 2.39 0.39 to 2.39 0.39 to 2.39 0.39 to2    1.5 K₂O 0.07 to 0.25 0.07 to 0.1  0.07 to 0.1  0.07 to 0.1    0.1to 0.25  CaO   0 to 1.27   0 to 1.27   0 to 0.04 0 to 1.27 0 to 0.5 MgO  0 to 2.6   0 to 2.6   0 to 2.6 0 to 2.49 1.6 SO₃   0 to 0.19   0 to0.19   0 to 0.09 0 to 0.19 0 Total iron as Fe₂O₃  0.05 to 0.061  0.05 to0.061  0.05 to 0.061 0.05 to 0.061   0.05 MnO₂   0 to 0.02   0 to 0.02  0 to 0.02 0 to 0.02 0 ZrO₂   0 to 1.5   0 to 1.19 0 0 to 1.19 0 to 1.5Total tin as SnO₂   0 to 0.5   0 to 0.5 0 0 to 0.5  0.5 TiO₂   0 to 1.89  0 to 1.89   0 to 0.01 0 to 1.89 0 to 0.5 P₂O₅   0 to 1.06   0 to 1.060 0 to 1.06 0 to 1   ZnO   0 to 0.5   0 to 0.5 0 0 to 0.5  0 to 0.5 B₂O₃0 to 2 0 to 2 0 to 2 0 to 1.55 0 to 0.5 NF (Al₂O₃ + ZrO₂) 18.26 to 28.5121 to 26 24.87 to 26   21 to 23.51 22 to 23.5 RO (CaO + MgO) 0.04 to2.64 0.04 to 2.64 0.04 to 2.64 0.04 to 2.54   1.6 to 2.1   R₂O (Na₂O +K₂O) 0.49 to 2.49 0.49 to 2.49 0.49 to 2.49 0.49 to 2.07   1.6 to 1.75 

As can be appreciated, the ingredients having a range with the lowerlimit “0” are considered ingredients that can be omitted from thecomposition. Further in the practice of the invention, the narrow rangesor single wt % values can be expanded to be within or be any of theranges discussed for that ingredient. By way of illustration, but notlimiting to the invention, Fe₂O₃ in the range of 0.05 to 0.061 or the wt% of 0.050 can be expanded to a range of less than 1.25 wt %, e.g. inthe range of 0 wt % or a tramp amount to 1.25 wt %, preferably less than1.00 wt %, e.g. in the range of 0 wt % or a tramp amount to 1.00 wt %,and more preferably less than 0.50 wt %, e.g. in the range of 0 wt % ora trace amount to 0.50 wt %.

Nonlimiting embodiments of the invention for glasses having ingredientsin the ranges listed in Column 5 above, and which are heated for 16 to22 hours at 374° C. in a molten bath of 100% pure grade sodium nitrateinclude, but are not limited to, glasses having at least one of thefollowing properties: a weight gain in the range of 1.4 to 2.5 mg/cm², acase depth in the range of 7 to 13.3 mils, an AMOR in the range of 72 to78.6 KPSI, an MOR in the range of 76 to 106 KPSI, a center tension inthe range of 6.1 to 10.3 KPSI, a CTA in the range of 554 to 1033pounds/inch, a Vickers hardness in the range of 600 to 614 kg/mm²,and/or a Knoop hardness in the range of 503 to 526 kg/mm². Further,nonlimiting embodiments of the invention for glasses having ingredientsin the ranges listed in Column 5 above, and which are heated for 16 to22 hours at 405° C. in a molten bath of 100% pure grade sodium nitrateinclude, but are not limited to, glasses having at least one of thefollowing properties: a weight gain in the range of 1.8 to 2.7 mg/cm², acase depth in the range of 11.5 to 17.4 mils, an AMOR in the range of 77to 78 KPSI, an MOR in the range of 71 to 112 KPSI, center tension in therange of 6.1 to 10.3 KPSI, a CTA in the range of 749 to 1080pounds/inch, a Vickers hardness in the range of 598 to 606 kg/mm²,and/or a Knoop hardness in the range of 505 to 522 kg/mm². The aboveranges were developed from the data on Table 3.

The chemical strengthened glasses of invention can be used in themanufacture of aircraft transparencies. The glass can be part of alaminated transparency or a monolithic transparency. The inventioncontemplates shaping the glass, chemically tempering the shaped glass,using one or more pieces of the shaped glass with one or more pieces ofplastic and/or standard soda-lime-silica glass to form a laminate andsecuring a mounting frame to the laminate to provide an aircrafttransparency and securing the frame of the transparency to the aircraftbody. The foregoing identified processes are well known in the art andno further discussion is deemed necessary.

The invention contemplates applying a coating to one or both the majorsurfaces of the chemically strengthened glass before or afterfabricating the strengthened glass into an article, e.g. a transparencyfor a vehicle. For example, but not limiting to the invention, the outersurface of the article can include a self cleaning coating, e.g. of thetype disclosed in U.S. Pat. No. 6,027,766 and/or sold by PPG Industries,Inc under their trademark SUNCLEAN and/or a hydrophobic coating of thetype disclosed in U.S. Pat. No. 6,025,025 sold by PPG Industries Inc.under the trademark SURFACE SEAL, which patents are herby incorporatedby reference. Further the invention contemplates applying electricallyheatable coating or solar control coatings of the type used in the art,e.g. U.S. Pat. No. 5,364,685, which patent is hereby incorporated byreference, to one or both the major surfaces of the glass of the presentinvention.

Based on the description of the embodiments of the invention, it can beappreciated that this invention is not limited to the particularembodiments disclosed, but it is intended to cover modifications thatare within the spirit and scope of the invention, as defined by theappended claims.

1. An aircraft transparency comprising a glass piece, the glass piece comprising: a chemically tempered first major surface and an opposite second major surface; a case depth, wherein the case depth is defined as a distance from the first major surface of the glass piece toward the second major surface, the distance ending at a position within the glass piece at which the glass piece has zero stress, and a tensile stress zone within the glass piece at a depth from the first major surface greater than the distance of the case depth from the first major surface, wherein glass in the tensile stress zone has a glass composition comprising: Ingredient Percent by weight SiO₂ 60 to 75; Al₂O₃ 18 to 29 Li₂O 3 to 9; and ZrO₂ 0 to 3;

wherein percent by weight (“wt %”) of Al₂O₃ +ZrO₂ is in the range 18 to 29 wt %, and wherein the glass in the tensile stress zone has at least one of the following properties: (a) a log 10 viscosity temperature of at least 1280° F. (694° C.) and (b) a liquidus temperature of at least 2350° F. (1288° C.), and has an unabraded modulus of rupture in the range of 71 to 112 thousand pounds per square inch (“ KPSI”).
 2. The aircraft transparency according to claim 1, wherein the glass in the tensile stress zone has a log 10 viscosity in the temperature range of 1328° F. (720° C.) to 1499° F. (815° C.); a liquidus temperature in the temperature range of 2437° F. (1336° C.) to 2575° F. (1413° C.), and a log 7.6 softening point temperature in the temperature range of 1544° F. (840° C.) to 1724° F. (940° C.).
 3. The aircraft transparency according to claim 2, wherein the unabraded modulus of rupture is in the range of 76 to 106 KPSI and wherein the glass piece has at least one of the following properties: a case depth in the range of 7 to 13.3 mils, an abraded modulus of rupture in the range of 72 to 78.6 thousand pounds per square inch (“KPSI”), a center tension in the range of 6.1 to 10.3 KPSI, a center tension area in the range of 554 to 1033 pounds/inch, a Vickers hardness 1000 in the range of 600 to 614 kilograms per square millimeter (“kg/mm²”), and a Knoop hardness 1000 in the range of 503 to 526 kg/mm².
 4. The aircraft transparency according to claim 2, wherein the glass piece has at least one of the following properties: a case depth in the range of 11.5 to 17.4 mils, an abraded modulus of rupture in the range of 77 to 78 thousand pounds per square inch (“KPSI”), center tension in the range of 6.1 to 10.3 KPSI, a center tension area in the range of 749 to 1080 pounds/inch, a Vickers hardness 1000 in the range of 598 to 606 kilograms per square millimeter (“kg/mm²”), and a Knoop hardness 1000 in the range of 505 to 522 kg/mm².
 5. . The aircraft transparency according to claim 1, wherein the case depth is a first case depth and further comprising a second case depth, wherein the tensile stress zone is between the first and the second case depths, and the second case depth is defined as a distance from the second major surface of the glass piece toward the first major surface and ending at a position within the glass piece at which there is zero stress.
 6. The aircraft transparency according to claim 5, wherein the glass piece submerged into a container of boiling 0.50 wt % sulfuric acid for 30 minutes has weight loss in the range of 0.002 to 0.014 milligrams per square centimeter-hour.
 7. The aircraft transparency according to claim 1 further comprising a mounting frame wherein the glass piece is secured in the mounting frame.
 8. The aircraft transparency according to claim 7, wherein the case depth is a first case depth and further comprising a second case depth, wherein the tensile stress zone is between the first and the second case depths, and the second case depth is defined as a distance from the second major surface of the glass piece toward the first major surface and ending at a position within the glass piece at which there is zero stress.
 9. An aircraft transparency comprising a glass piece, the glass piece comprising: a chemically tempered first major surface and a chemically tempered opposite second major surface; a first case depth and a second case depth, wherein the first case depth is defined as a first distance from the first major surface of the glass piece toward the second major surface, the first distance ending a first position within the glass piece at which there is zero stress and the second case depth is defined as a second distance from the second major surface of the glass piece toward the first major surface, the second distance ending a second position within the glass piece at which there is zero stress, wherein the first position is spaced from the second position, and a tensile stress zone defined as interior glass, wherein the interior glass is within the glass piece between the first and the second positions, wherein the interior glass has a glass composition comprising: Ingredient Percent by weight SiO₂ 60 to 75; Na₂O 0.39 to 2.00 Al₂O₃ 18 to 28; and Li₂O 3 to 9;

wherein the unabraded modulus of rupture is in the range of 71 to 112 thousand pounds per square inch (“KPSI”) and wherein the glass composition has (a) a log 10 viscosity temperature of at least 1413° F. and (b) a liquidus temperature of at least 2436° F.
 10. The aircraft transparency according to claim 9, wherein the glass within the first case depth has a lower concentration of lithium than the interior glass.
 11. The aircraft transparency according to claim 9, wherein the first distance is in the range of 7 to 16.5 mils.
 12. The aircraft transparency according to claim 9, wherein the glass composition of the interior glass comprises: Ingredient Percent by weight SiO₂ 64.83 to 69.29; Al₂O₃ 21.97 to 23.51; Li₂O 4.94 to 6.50; K₂O 0.07 to 0.10;

where the combined percent by weight (“wt %”) of Na₂O and K₂O is in the range of 0.49 to 2.07 wt %.
 13. The aircraft transparency according to claim 12, wherein the glass piece has a center tension in the range of 5,700 to 14,900 pound per square inch.
 14. The aircraft transparency according to claim 12, wherein the glass piece has a center tension area in the range of 632 to 979 pounds per inch.
 15. The aircraft transparency according to claim 12, wherein the glass piece has an unabraded modulus of rupture in the range of 76,000 to 106,000 pounds per square inch.
 16. The aircraft transparency according to claim 12, wherein the glass piece has a Vickers's hardness 1000 in the range of 586 to 614 kilograms per square millimeters measured on one of the major surfaces.
 17. The aircraft transparency according to claim 9 further comprising a mounting frame wherein the glass piece is a layer of a laminate and the laminate is secured in the mounting frame.
 18. The aircraft transparency according to claim 1 further comprising Na₂O in the range of 0.39 to 2.00 wt %.
 19. The aircraft transparency according to claim 1 further comprising Na₂O in the range of 0.05 to 1.5 wt %.
 20. The aircraft transparency according to claim 9, wherein Na₂O is in the range of 0.05 to 1.5 wt %. 